Diesel tank filling with grease.

Purging too long would fill your grease tank with diesel, not your diesel tank with grease.
The valves are getting power according to your readings. If you can get someone to help you, have them put their hands on the valves with the car shut off. Switch to grease and they should hear and feel both valves click. When you put the switch back to the center position both valves should click again, indicating that the NO ports are opening again and the NC ports are closing. Hit the purge from the center position and your assistant should only feel the return valve click on and off, and not the supply valve as it supplies diesel during purge. You know the valves are getting power but you still want to make sure they are opening and closing.
Another thing you can do is take two hoses off the return valve. Leave the hose on that is stamped COM and disconnect both the NC and the NO hoses and let them drip into a container. start the car (if it will start) and let it idle. the return fuel from the NO hose should be coming out into the container. Switch to grease and fuel should immediately start flowing out of the hose from the NC port into the container. If not, your valves are probably stuck.
I'm guessing you know that grease is getting into the diesel tank because your diesel gauge was rising? Because hard starting could be something else altogether. Air leaks cause this problem too.